New in version 2.0.
Binomial coefficient of n choose k:
abjad> from abjad.tools import mathtools
abjad> for k in range(8):
... print k, '\t', mathtools.binomial_coefficient(8, k)
...
0 1
1 8
2 28
3 56
4 70
5 56
6 28
7 8
Return positive integer.